Ezine is a short for electronic magazine, or online newsletter. An ezine is sent by different website and business owners in the form of an email to their subscribers. The Ezine is generally related to a particular niche or a topic. As you probably know by now, one of the best ways to market online is to send your advertisement by email to a list of targeted opt-in subscribers (the money is in the list). It is important that the list will be of opt-in subscribers; you wouldn't like your ads to appear in any spam mail. Ezines are sent only to this type of subscribers, people who are interested in receiving it. That's why it's not spam and that's why if you choose an ezine related to your niche, your readers would be targeted audience. Ezine ads are a way for those who don't have their own list of subscribers, to still market in this effective way.

1. Solo ad - is an email sent to the ezine members, containing your offer alone. The length of this kind of ad can be as much as you wish. Since there is only your offer, it's the most responsive type of ad you can have on ezines (and therefore the most expensive one as well). 
2. Classified ad - is a short ad that appears on the ezine or classified ads websites. It generally contains a few lines that should concisely describe your offer and a link to your website or your affiliate link. It is less effective than a solo ad, and therefore cheaper, but you can get results if you consistently use it. There are 3 types of classified ads according to their placement in the ezine – Top, Middle or Bottom Sponsor. Among the classified ads Top Sponsor is of course the most effective, since it appears at the top of the ezine, and therefore the first thing the reader see. Even if he will not read the rest of the newsletter, he will certainly see this ad.

Ezine advertising can be very cost effective, but in order to be so, you should have (or develop) some skill in copywriting, or you should get someone else to write your ads for you. If you are an affiliate marketer, you may use promotion material that was written by the product's owner. In such a case, it's better to change a little bit the original material, so that your ad will be unique.

I would love to give you a list of ezines you could use, the problem is that there are unlimited ezines in unlimited niches, therefore it's impossible to do so, but you can find ezines by conducting a search on a search engine for "ezine directories" or "ezine + your niche". No matter what your niche is, it is important that your website or your link will receive targeted traffic -- people who get there because they're really interested in what you have to offer -- therefore don't put your ad on any ezine just because it has many members, or because the ads are cheaper, look for ezine that will best fit your website or the product you're promoting.

Although you can manage to find ezines and write your advertising for free, if you're just at the beginning of your journey in internet marketing it can be hard to figure out all by yourself. If you need help in writing, or consulting, and a very good directory I would suggest The Directory of Ezines. This great service gives you not only a list of ezines by categories, but also consultation with an expert e-marketer, training on ezine marketing, 3 free custom ads for any product or website you want, free ads on their websites, free submission to their article directory (that is promoted to ezines' owners), and an easy way to find ezines that offer ads, offer joint venture (publishing your ad for a percentage of your profit) or offer the possibility to publish your article without having to compete with thousands of other articles as it is in article directories. I would suggest that you take a look.